Hi
I have not experience with conversion from AD&D 1.0 or 2.0 to 3.5. I know nothing about AD&D 1.0 and 2.0. But I read that in 1.0 XP reward for killed monsters is very low in comparsion with 3.5.
So, if 3.5 version has the same number of monster of the same monsters, PCs will gain levels...